This invention relates generally to powder preform consolidation processes, and more particularly to such processes wherein substantially texture free nanocrystalline crystalline materials, oxide dispersion strengthened, are produced or formed.
One of the most promising methods to improve the mechanical and physical properties of aluminum, as well as many other materials, is that of nanocrystalline engineering. Significant interest has been generated in the field of nanostructured materials in which the grain size is usually in the range of 1-100 nm. More than 50 volume percent of the atoms in nanocrystalline materials could be associated with the grain boundaries or interfacial boundaries of nanocrystalline materials when the grain size is small enough. A significant amount of interfacial component between neighboring atoms associated with grain boundaries contributes to the physical properties.
Designers of modern commercial and military aerospace vehicles and space launch systems are constantly in search of new materials with lower density, greater strength, and higher stiffness. New technical challenges, such as those presented by the Integrated High Payoff Rocket Propulsion Technology (IHPRPT) program, are ideal proving grounds for advanced materials. To meet these challenges much effort has been directed toward developing intermetallics, ceramics and composites as structural and engine materials for future applications. For structural airframes aluminum alloys have long been preferred for civil and military aircraft by virtue of their high strength-to-weight ratio, though the use of composite materials, particularly for secondary structures, is rapidly increasing. Nearly 75% of the structure weight of the Boeing 757-200 airplane is comprised of plates, sheets, extrusions, and forgings of aluminum alloys. Therefore, further improving the physical and mechanical properties of aluminum alloys, while simultaneously decreasing their weight, will have a significant effect on the entire aerospace industry.
The sudden burst of enthusiasm towards nanocrystalline materials stems not only from the outstanding properties that can be obtained in materials, such as increased hardness, higher modulus, strength, and ductility, but also from the realization that early skepticism about the ability to produce high quality, unagglomerated nanoscale powders was unfounded. Additionally, the ability to synthesize an entirely new generation of composites, nanocrystalline metal matrix composites, has further sparked this enthusiasm.
Potential applications for nanocrystalline materials, including their composites, span the entire spectrum of technology, from thermal barrier coatings for turbine blades, to static rocket engine components such as high pressure cryogenic flanges (Integrated High Payoff Rocket Propulsion Technology), to electronic packaging, to static and reciprocating automotive engine components. Although structures and mechanical properties of nanocrystalline aluminum alloys have been reported by several researchers, most of the materials produced have been thin ribbons or very small, pellet type powder samples. Cost effective, bulk powder production and near-net-shape product manufacturing is virtually non-existent and offers a significant opportunity in the commercial marketplace. The routine manufacture of functional, near-net-shape components that also maintain the nano-scale morphology has not yet been accomplished.
It is a major object of the invention to provide a powder metallurgy (PM) process to achieve formation of nanocrystalline aluminum and a substantially texture free microstructure. In accordance with the process of the invention, employing a fluidized bed chemical vapor deposition (CVD) technique, several nanophase Aluminum/Silicon Carbide (SiCp)/Aluminum oxide, dispersion strengthened metal matrix composite (MMC) powders were produced. The powders were consolidated to full density in seconds via the herein disclosed solid-state consolidation technology. Applicants"" solid-state powder metallurgy (P/M) consolidation enabled retention of the nanocrystalline aluminum while simultaneously producing a virtually texture free microstructure. Increases of 30% in flexure modulus and 25% in flexure strength over commercially available 25 v/o (volume per-cent) SiC composites have been demonstrated. Similarly, the specific moduli of both the 25 v/o and 35 v/o SiC CVD coated and forged powders demonstrated increases of 25% and 50% respectively when compared to conventionally produced aluminum MMC products. Near net shape P/M forging of the nanophase MMC powders into prototype structural components was also demonstrated.
Basically, the process includes the steps:
a) pressing the powder into a preform, and preheating the preform to elevated temperature,
b) providing a bed of flowable pressure transmiting particles,
c) positioning the preform in such relation to the bed that the particles encompass the preform,
d) and pressurizing the bed to compress said particles and cause pressure transmission via the particles to the preform, thereby to consolidate the preform into a desired shape.
As will be seen, such pressurizing may be carried out to maintain or preserve the nanocrystalline aluminum grain size, thereby to develop a substantially texture free microstructure at metallic grain boundaries.